The Japanese document No. 5116670 describes a process for forming an inner cavity in the solid body of a crank arm by extrusion. According to this proposal, the cavity extends up to one end of the crank arm. Consequently, in order to obtain the threaded hole for fixing of the pedal it is necessary to provide an insert fixed in an end area of the crank arm. This solution presents structural strength problems in so far as it gives rise to a basically tubular structure opened at one end. This presents poor resistance to torsional and bending stresses, in particular owing to the fact that the point of application of a load, represented by the area of connection to the pedal, is provided in a hollow part. The U.S. Pat. No. 6,058,803 describes a process for producing a hollow crank arm by casting, in which a sand core is positioned in a die and the molten metal is poured into the die and solidified in order to form a crank arm body with an elongated cavity having a blind bottom and an end that opens to the outside of the hole which is provided for fixing the pedal. Also this solution presents strength problems because the cavity opens into the hole for connection of the pedal and thus weakens that area.